Add sidecar serialisation for the edit graph

Sidecars are the only thing in the trust path: the catalog can be rebuilt
from them, so they carry the edit graph and its version in a form that
survives a schema change.

Assisted-by: LLM

core/dr-pipeline/src/lib.rs

@@ -37,6 +37,7 @@ pub mod graph;
 pub mod lens;
 pub mod operation;
 pub mod ops;
+pub mod sidecar;
 
 pub use descriptor::{
     LocalizedKey, OpDescriptor, OpId, ParamDescriptor, ParamId, ParamKind, Presentation, Scale,
@@ -49,6 +50,7 @@ pub use operation::{
     compose, compose_with_framing, Affects, ComposedShader, Helper, Operation, Uniform,
     RESERVED_UNIFORM_FIELDS,
 };
+pub use sidecar::{Sidecar, Version};
 
 #[cfg(test)]
 mod tests {